Modern circuit protection increasingly calls for circuit breakers equipped with so-called "static" or electronic trip units in lieu of the traditional thermal-magnetic or dual magnetic trip units for achieving automatic opening of the breaker contacts in response to overcurrent conditions ranging from light overload to heavy short circuit. Static trip units are found to be more versatile and precise in terms of selectively establishing multiple overcurrent pickup levels and trip time delays. For example, currently available static trip units have the capability of selectively establishing coordinated long time delay, short time delay and instantaneous overcurrent pickup levels, as well as different tolerance bands of time delays. As a consequence, the trip settings of a static trip circuit breaker can be readily tailored to the load so as to provide proper protection and yet avoid vexations nuisance tripping.
Another reason for the current popularity of static trip circuit breakers is the increasing demand for ground fault protection. Since response to a ground fault condition is best handled electronically, it becomes quite practical to integrate the ground fault trip function into an overcurrent responsive electronic trip unit in contrast to providing an electronic ground fault trip unit plus a traditional electromechanical trip unit.
Once a static trip circuit breaker goes into the field, it is desirable to verify its continuing capability to provide the full measure of circuit protection intended for a particular application. To this end, field test sets of two types are available. In one type of test set, a high current of overcurrent proportions is passed through each of the breaker poles, to verify that the breaker will trip. In the second type, low current, fault simulating signals are injected into the secondary circuits of the breaker phase current monitoring transformers which are connected as inputs to the static trip unit. If the trip unit is functioning properly, it will process these fault simulating signals pursuant to initiating a trip function as though corresponding high currents of overcurrent proportions actually flowed through the breaker poles, i.e., the primaries of the breaker current transformers.